The present invention relates to an all-wheel drive for a motor vehicle comprising a first transmission line, which starts out from a change-speed gearbox and leads to a first vehicle axle and a second transmission line, which branches off from the first transmission line and leads to the second vehicle axle.
An all-wheel drive such as the type described above in German Patent Document 35 07 490. From a transmission line leading to the front axle, which is rigidly driven by a front engine, a transmission line branches off to the rear axle and can be connected by way of an all-wheel locking system. The locking system used in this case comprises a planetary transmission which provides the direct through-drive and a centrifugal brake applied to its planet carrier. On the planetary transmission, the ratio of the input shaft to the output shaft deviates only slightly from 1:1; however the ratio of the input shaft to the planet carrier is very high, for example, 16:1. The planetary transmission comprises an input sun gear which is non-rotatable with respect to the input shaft; an output sun gear which is disposed coaxially next to the input sun gear, is connected with the output shaft and has a slightly lower number of teeth; as well as the planet gears which mesh with both sun gears simultaneously and are disposed on the planet carrier. On the planet carrier, bearing bolts are mounted on which brake shoes are disposed in such a manner that they act as centrifugal weights and, by means of their brake linings, can rest against a stationary brake drum. When a rotational speed difference occurs between the front axle and the rear axle, a locking moment is generated which rises progressively with the rotational speed difference and results in an optimal traction of the vehicle. Beyond that, however, in the case of motor vehicles to be newly developed, more and more requirements come to the foreground with respect to directional control, ABS-capability, and driving safety in general, and also in marginal situations.
It is an object of the invention to provide an all-wheel drive which permits the implementation of an optimal handling with respect to driving dynamics in the case of braking operations, cornering and when starting and accelerating the motor vehicle.
These and other objects are achieved by the present invention which provides an all-wheel drive for a motor vehicle comprising a drive shaft and an output shaft, and a first transmission line which is coupled to a change speed gearbox and a first vehicle axle. A second transmission line branches off at a branch-off point from the first transmission line and leads to a second vehicle axle. A permanently operative all-wheel locking system connects the second transmission line between the drive shaft and the output shaft as a function of a rotational speed difference. A shiftable locking coupling is coupled to the wheel locking system that is operable to lock the all-wheel locking system in such a manner that the second vehicle axle is driven rigidly by the second transmission line. A shiftable separating coupling is inserted in the first transmission line according to the power flux behind the branch-off point to the second transmission line.
When a shiftable separating coupling is inserted into a first transmission line leading to the rear axle behind the branch-off point to the second transmission line leading to the front axle, it becomes possible, by opening up the separating coupling, to separate the rear axle completely from the front axle and from the drag moment of the engine. This separation takes place when the vehicle brake is actuated controlled by the ABS signal in approximately 130 ms. Thus, an optimal ABS-compatibility and directional control is achieved. In the coasting operation of the motor vehicle, the separating coupling does not open up so that an engine braking can take place with respect to all 4 wheels.